Post Affiliate Pro and Membership ???

I have membership working with paypal express on my WordPress Site.

I have Post Affiliate Pro tracking referrals and sales on all my sites. Except my wordpress site running membership.

Post Affiliate Pro integration for paypal express (ipn being used by another script) instructs teh following code to be added to the paypal button.

<input type="hidden" name="custom" value="" id="xxxxxxxxxxx">
<script id="xxxxxxxxxxx" src="http://urlgoeshere.com" type="text/javascript">
</script>

How can I add this to membership????? So I can have all my sites and users on the same affiliate tracking software?

Any other ideas on how to integrate PAP with membership